Several encoding techniques to compress an amount of data of multi-channel audio signals are disclosed. One of such techniques is MPEG surround standardized by moving picture experts group (MPEG).
In MPEG surround, audio signals (time signal) of 5.1 channel as an encoding target are time-frequency transformed, and resulting frequency signals are down-mixed to generate 3-channel frequency signals. The 3-channel frequency signals are down-mixed again to calculate 2-channel frequency signals corresponding to stereo signals. The 2-channel frequency signals are then encoded through advanced audio coding (MC) method, and spectral band replication (SBR) coding method. In MPEG surround, spatial information representing spreading of sound or echolocation is calculated in the down-mixing from the 5.1 channels to 3 channels, and in the down-mixing from 3 channels to 2 channels. The spatial information is also encoded together.
In the MPEG surround, stereo signals resulting from down-mixing multi-channel audio signals and the spatial information that is relatively small in data quantity are encoded. The MPEG surround provides a compression rate higher than when channel signals contained in the multi-channel audio signal are independently encoded.
In the MPEG surround, a prediction coefficient is used to encode the spatial information that is calculated in the generation of 2-channel stereo frequency signals. The prediction coefficient is a coefficient that is used to obtain 3-channel signals by up-mixing the 2-channel signals subsequent to the down-mixing. More specifically, the coefficient is used to predict one of the 3-channel signals based on the two other channel signals. The up-mixing is described with reference to FIG. 1.
As illustrated in FIG. 1, the down-mixed 2-channel signals are denoted by an l vector and an r vector. A signal resulting from up-mixing the 2-channel signals is denoted by a c vector. In the MPEG surround, the vector c is predicted using prediction coefficients c1 and c2 in accordance with expression (1):c=c1l+c2r  (1)
Values of a plurality of prediction coefficients are stored on a table referred to as a “code book”. The code book is used to increase a use bit rate. In the MPEG surround, each of c1 and c2 takes all values within a range of from −2.0 or larger to +3.0 or smaller in steps of 0.1, namely, combinations of 51×51 are stored on the code book. If the combinations of prediction coefficients are plotted on the orthogonal coordinate system having c1 and c2 as two coordinate axes, 51×51 grid points are obtained as the code book.
A related art technique is available to select a combination of prediction coefficients from the code book. According to the technique, an error defined by a difference between a channel signal prior to predictive encoding and the channel signal subsequent to the predictive encoding is calculated using all combinations of prediction coefficients stored on the code book, and a combination providing a minimum error is selected. Japanese National Publication of International Patent Application No. 2008-517338 discusses a technique of selecting a combination having a minimum error through least square algorithm.
In another related art techniques, additional information is embedded in data. For example, Japanese Laid-open Patent Publication No. 2009-213074 discloses a technique of embedding digital watermark information in encoded data. According to the disclosed technique, compression encoded data is re-encoded using an encoding control parameter different from an encoding control parameter that has been used in the compression encoded data. Japanese Laid-open Patent Publication No. 2000-013800 discloses a technique of hiding encryption information in image data. According to the disclosed technique, a predictive mode signal of predictive image data is corrected in accordance with the encryption information, and encoding is performed in response to a predictive mode signal subsequent to the correction. During decoding, the corresponding encryption information is extracted first from the predictive mode signal.